Re: [resend Patch v3 1/2] kaslr: check if kernel location is changed

[Baoquan He]

On 10/15/14 at 11:37am, Baoquan He wrote:
> On 10/14/14 at 08:49am, Vivek Goyal wrote:
> > On Mon, Oct 13, 2014 at 01:22:42PM -0400, Vivek Goyal wrote:
 
> > > Agreed. On x86_64, we should be able to randomize virtual address space
> > > and physical address space independently. And in that case whole of
> > > the physical memory should be available for a possible location for
> > > kernel. (As opposed to a small limit (I guess 1GB) now)
> 
> It can be done to randomize virtual address space and physical address
> space independently. But limited by the 2G of kernel text mapping and
> module mapping virtual address space, virtual address can be randomized
> in (0x1000000, 1G) range. While physical address can be randomized in
> (0x1000000, 4G) according to the identity mapping of normal kernel. Then
> phys_base still stores an relative value, a different offset than before.
> 
> This can be easily implement. One thing is still there's a limit for
> physical addr randomization, only below 4G. So I am wondering if we can
> extend the identify mapping to complete mapping of 48 bit, using 1G page
> frame. This can make physical addr be randomized to anywhere.
> 
> So now there may be 3 options:

Hi hpa,

About kaslr, do you have a suggestion or plan? Should we fix bug based
on current code, or any other solutions?

Thanks
Baoquan


> 
> 1) Fix this bug in current kaslr. Since when randomize the new kernel
> location in choose_kernel_location(), cmdline options has been checked
> strictly, e.g if nokaslr is specified, it's safe to do the kernel
> location randomization. Then in handle_relocations(), we only need to
> check if the kernel location is changed, comparing with kernel loaded
> addr. If changed, kaslr is done, let's do the relocation handling. If
> not changed, no kaslr id done, just skip the relocation handling like
> before.
> 
> 2) randomize the virtual addr space and physical addr space
> independently. But physical addr space must be below 4G.
> 
> 3) extend the identity mapping to 48bit of addr space. Then we can
> randomized the virtual addr space in (0x1000000, 1G) and physical addr
> space in (0x1000000, real physical memory end).
> 
> If option 3 is doable, it's the best. If not, I think bug fix should be
> better.
> 
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